The overall objective of this project is to elucidate the temporal aspects of stimulus coding in the visual cortex of the monkey. Specifically, the project investigates the hypothesis that short-latency activity evoked in a single visual cortex cell signals the detection of a stimulus while later activity codes brightness information. Metacontrast masking is used as a tool to test the hypothesis. In metacontrast, the apparent brightness of a briefly flashed pattern is reduced if it is followed by another flashed pattern having contours in common with the first. Thus metacontrast is used as a methid of separating stimulus intensity from perceived brightness. The fact that the delay of the second flash can be longer than the post-stimulus latency of the early cortical response to the first flash means that later activity must be involved in making brightness judgments. Monkeys will perform a brightness discrimination task while one of the stimuli to be compared is being masked by metacontrast. At the same time, recordings from cells in the striate cortex will be made. This will allow a close comparison of psychophysical brightness judgment with electrophysiological data. If the hypothesis is accurate, brightness judgments will be related to late, but not early, activity.